The invention relates to an internal planar antenna structure in small-sized radio apparatus such as mobile phones.
In portable radio apparatus it is very desirable that the antenna be placed inside the covers of the apparatus, for a protruding antenna is impractical. In modem mobile stations, for example, the internal antenna naturally has to be small in size. This requirement is further emphasized as mobile stations become smaller and smaller. Furthermore, in dual-band antennas the upper operating band at least should be relatively wide, especially if the apparatus in question is meant to function in more than one system utilizing the 1.7-2 GHz band.
When aiming at realizing a small-sized antenna the most common solution is to use a PIFA (planar inverted F antenna) structure. The radiating element in a PIFA may form a continuous plane, producing an antenna of one useful operating band. The radiating element may also have a slot in it which divides the element, viewed from the feed point, into two branches so that an antenna of two useful operating bands can be produced. The latter structure is more interesting since mobile stations functioning in two systems utilizing different frequency bands have become popular. The dual-band structure also provides for a suitable framework for the description of the present invention.
FIG. 1 shows an example of a prior-art dual-band PIFA. In the Figure there can be seen the frame 120 of the apparatus in question which is drawn horizontal and which functions as the ground plane of the antenna. Above the ground plane there is a planar radiating element 110 which is supported by insulating pieces, such as 105. Between the radiating element and ground plane there is a short-circuit piece 102. The radiating element 110 is fed at a point F through a hole 103 in the ground plane. In the radiating element there is a slot 115 which starts from the edge of the element and extends to near the feed point F after having made two rectangular bends. The slot divides the radiating element, viewed from the feed point F, into two branches A1 and A2 which have different lengths. The longer branch A1 comprises in this example the main part of the edge regions of the radiating element, and its resonance frequency falls on the lower operating band of the antenna. The shorter branch A2 comprises the middle region of the radiating element, and its resonance frequency falls on the upper operating band of the antenna.
In the structure according to FIG. 1 the slot between the branches of the radiating element is relatively narrow so that there exists an electromagnetic coupling of considerable magnitude between the branches. As a consequence, the electrical length of the branches is greater than the mechanical length. This, in turn, results in the advantage that an antenna functioning in given frequency bands becomes smaller compared to a corresponding antenna without said electromagnetic coupling. A disadvantage of the coupling is, however, that the electrical characteristics of the antenna are affected; for example, the bandwidth becomes smaller and the losses become greater. Conversely, if the slot in the radiating element is made wider, the electrical characteristics of the antenna will improve, but the antenna has to be made bigger. As is known, the frequency bands may also be made wider by increasing the distance between the radiating element and ground plane, but this arrangement, too, has the disadvantage of making the antenna bigger.
The object of the invention is to reduce said disadvantages associated with the prior art. The structure according to the invention is characterized by what is expressed in the independent claim 1. Some preferred embodiments of the invention are presented in the other claims.
The basic idea of the invention is as follows: a layer of dielectric material, the dielectric constant of which is relatively high, is arranged outwards of the plane of the outer surface of the radiating element of a PIFA. The layer is located so as to cover at least the areas in which the electric field is the strongest when the antenna resonates. In the case of a dual-band antenna the slot of the radiating element is made advantageously so wide that the effect of the coupling between the branches of the element is small.
The addition of dielectric material has the known effect of shifting down the resonance frequency or frequencies of the antenna so that in order to retain a given resonance frequency the size of the resonating element has to be reduced. On the other hand, the addition of dielectric material at advantageous locations has the effect of keeping the impedance of the antenna close to the nominal value over a wider frequency range, which means a greater bandwidth. This is based on directing the stray flux flowing outside the space between the radiating element and ground plane onto a wider route. As was described above, the widening of the slot of the radiating element results in the improvement of the electrical characteristics of the antenna but, on the other hand, it also results in the fact that the antenna has to be made bigger if the resonance frequencies are to be located as desired.
By suitably combining addition of dielectric material xe2x80x9con topxe2x80x9d of the radiating element and widening of the slot in the element, the antenna can be made smaller and at least as good in its electrical characteristics as a corresponding prior-art antenna. Alternatively, the electrical characteristics of the antenna can be substantially improved without increasing the size of the antenna. In the latter case, the effects on the size of the antenna of the addition of dielectric material and widening of the slot of the radiating element are opposite to each other. Naturally, in accordance with the invention, a structure may be arranged which falls in or outside the intermediate area between said two cases. In addition, the invention has the advantage that the structure according to it is simple and relatively low in manufacturing costs.